The present invention relates, in general, to novel optical signal processors, and, more particularly, to novel integrated optical signal processors which make use of the Bragg diffraction of guided light by magnetostatic waves.
A variety of optical signal processing and communications applications could benefit from optical modulation in the 1 to 20 GHz microwave region. Presently used approaches such as direct modulation of semiconductor lasers, traveling-wave electro-optic modulators, and acousto-optic diffraction are generally restricted to a few GHz at best. Traveling-wave electro-optic modulators can, under some circumstances, exceed 10 GHz but cannot generally be operated with a large time-bandwidth product (TBW) in tranverse one-dimensional modulator geometries. On the other hand, acousto-optic deflectors can offer a large time-bandwidth product, but are usually limited to center frequencies below a few GHz. In contrast, magneto-optic modulation by magnetostatic waves (MSW) appears to offer the potential of a large time-bandwidth product directly at microwave signal frequencies of up to 20 GHz or higher.
Bragg diffraction of optical beams by magnetostatic waves has been previously observed in bulk bar and rod ferrite geometries. This phenomenon has seen little further development primarily because the internal magnetic field of a bulk ferrimagnetic material in a uniform applied magnetic field tends to be spatially inhomogeneous due to shape factor demagnetization. This results in large spatial variations in the magnetostatic wave dispersion relationship and can even cause "turning points" which reflect the magnetostatic wave. Thus, the bulk geometry cannot be employed in practical optical signal processing devices.
Recently, thin ferrite films have been developed which exhibit numerous desirable magnetic characteristics such as a uniform internal magnetic field. Such thin films have been used to form bubble memory devices and for electronic magnetostatic wave delay-line devices. The magneto-optic and infrared transparency properties of these thin films have also been exploited to build integrated optical low-frequency modulators and non-reciprocal isolators utilizing optical Faraday rotation controlled by an external magnetic field. The present invention makes use of the desirable characteristics of thin ferrite films to form another type of optical signal processing device.